PROJECT SUMMARY: Hepatitis B virus (HBV) is a small, partially double-stranded DNA virus that causes acute and chronic hepatitis. An estimated 400 million people are chronically infected worldwide, many suffering early death due to liver failure and primary liver cancer (HCC). The chance of resolving HBV infection is age dependent: approximately 90% of neonatal infections become chronic, whereas at least 90% of adult infections are cleared spontaneously. It is generally accepted that a broad and diverse adaptive immune response is important in clearing acute HBV infection. However, why an individual generates, or fails to generate, a favorable response, and why this capability varies with age, is just beginning to be understood. The study of HBV immunopathogenesis has been limited because HBV only infects outbred species whose immune systems are difficult to examine and it does not infect mice, the species in which most of the tools to study immune mechanisms have been developed. My laboratory has developed transgenic mouse models of primary HBV infection that mimic key differences in HBV clearance and persistence in humans, including the age-related dichotomy in human HBV infection outcome. This model has allowed us to address fundamental questions in HBV biology: 1. Why is the immune response and disease outcome different depending on the age of the individual at the time of infection? 2. What are the immune mechanisms that facilitate viral control, and how do these differ from the immune mechanisms that lead to chronic viral infection? 3. Can immune modulation of pathways identified to be important in effective HBV immunity tilt ineffective immune responses toward viral control? Our collective data using this model and our correlative studies in humans, demonstrate that immune priming to HBV occurs in the liver, and that effective immune priming requires orchestrated formation of leukocyte clusters that are anchored by macrophages and monocytes. Effective HBV immunity requires hepatic TFH cell priming and IL-21 production in the liver, where it is essential for optimal generation of specific CD8+ T and B cell responses that are crucial for viral clearance. Furthermore, maturation of liver APCs and their age-dependent expression of the chemokine CXCL13 is crucial for B cell differentiation and class-switching, while age-dependent expression of the co-stimulatory ligand OX40L explains differences in TFH priming and IL-21 production in the liver. While these new data begin to help us formulate a new paradigm to explain age-dependent HBV persistence and to identify therapeutic targets, there are many unanswered questions related to the cells and pathways involved in the formation of hepatic leukocyte structures and the priming of effective immunity. Our proposal explores the hypothesis that a resident hepatic population of group 3 innate lymphoid cells (ILC3s) is important for the development of lymphoid organization and/or the priming of effective HBV immunity. In addition, we aim to target the development of hepatic ILC3s, and the expression of CXCL13 and OX40L, to tilt the immune response in young and ?chronic HBV mice? toward viral control and HBsAb seroconversion.